The objective is to develop descriptions of the mechanical properties of the left ventricle, that will allow further studies to be made of the manner in which the left ventricle interacts with the arterial system and with other components of the cardiovascular system. A significant level of understanding has been acquired from previous work. However, an important piece of information is missing because current descriptions of left ventricle mechanical properties are unable to accurately predict the time course of pressure, flow, and volume changes during a normal heart beat. An ability to make such predictions must be adopted as a rigorous standard to which our understanding of left ventricle function can be measured. Based on results from earlier experiments, a hypothesis is put forth for description of left ventricle mechanical properties. This hypothesis takes the form of a predictive model. It includes two properties that have been well described by others (elastance and resistance) and a third property (active and passive conductance) that is being introduced for the first time. A wide variety of experiments are proposed in the intact, open-chest animal and in the isolated heart. These experiments allow for critical testing of the hypothesis and for modifying and extending the conceptions that underly it. These experiments accomplish two more objectives: one will result in a technique that will allow definitive testing of any proposed mode of left ventricle function; the other will prepare for the application of a validated hypothesis of evaluation of left ventricle function in the intact animal. There will be two direct health-related outcomes from these studies. One, an accurate descriptor of left ventricle mechanical function will allow more refined discrimination between health and altered functional states than is now possible. Thus, the functional consequences of early myocardial disease may be assessed whereas now such consequences cannot be assessed. Two, with an accurate description of mechanical blood pumping, and not until such a description is forthcoming, it will be possible to control artificial hearts so that they pump blood in a manner analogous to a natural heart. This will be important to the long term success of these devices.